1. Field of the Invention
The present invention relates generally to an apparatus for controlling a lock-up clutch used in an automatic transmission of a motor vehicle, and more particularly to improvements in a control apparatus adapted to selectively engage or release such a lock-up clutch according to predetermined clutch-engaging and -releasing control patterns.
2. Discussion of the Related Art
An automatic transmission of a motor vehicle generally includes a fluid coupling device such as a torque converter connected to an engine of the vehicle, and a speed changing mechanism such as a planetary gear mechanism or a belt-and-pulley mechanism with variable-diameter pulleys, which is connected to the fluid coupling device. Power is transmitted through the fluid coupling device to the speed changing mechanism, and the speed ratio of the speed changing mechanism [input shaft speed/output shaft speed) is changed in steps or continuously. For improved fuel economy of the vehicle, some automatic transmissions of this type incorporate a lock-up clutch which is engaged to directly connect the speed changing mechanism to the engine, in certain running conditions of the vehicle.
Usually, a lock-up clutch is selectively engaged and released, according to predetermined clutch control patterns. FIG. 4 shows an example of the clutch control patterns for controlling a lock-up clutch used with a speed changing mechanism which has four forward drive positions. In this example, the clutch control patterns for selectively establishing the engaged and released positions of the lock-up clutch are defined in relation to two control parameters, that is, a currently detected running speed V of the vehicle and a currently detected accelerator pedal position (currently required output or currently detected load of the vehicle engine). In FIG. 4, the clutch control patterns are represented by solid lines indicative of clutch-engaging boundaries (OFF.fwdarw.ON boundaries) for operating the released lock-up clutch to the engaged position, and dashed lines indicative of clutch-releasing boundaries (ON.fwdarw.OFF boundaries) for operating the engaged clutch to the released position. The clutch-engaging and -releasing boundaries are set for each of the three forward drive positions (second-speed position, third-speed position and overdrive position) of the speed changing mechanism. When the speed changing mechanism is placed in the first-speed position having the highest speed ratio, the lock-up clutch is held in the released position. According to the clutch control patterns of FIG. 4, therefore, the lock-up clutch is selectively engaged and released depending upon the currently established position of the speed changing mechanism and the currently detected accelerator pedal position and vehicle speed. In the reverse position of the speed changing mechanism, the lock-up clutch is held in the released position.
For improving the adequacy of control of the lock-up clutch, it is considered to adjust or compensate the basic clutch control patterns according to compensating data maps, or use a large number of clutch control patterns, so that the lock-up clutch is more suitably controlled so as to meet the specific vehicle running condition of the vehicle such as the engine speed, and operating conditions of the accelerator and brake pedal positions. The compensation of the basic clutch control patterns or the use of many clutch control patterns permits optimum controlling of the lock-up clutch for enhanced driving comfort of the vehicle and reduced fuel consumption. However, this arrangement which uses many control parameters requires a considerably increased amount of control program data (control data maps), and therefore requires a large-capacity memory for storing such data, resulting in an increase in the cost of the control apparatus. Namely, the required amount of control program data increases in proportion to the number of control parameters raised to n-th power.
In view of the above drawback, the assignee of the present application developed a transmission control apparatus as disclosed in U.S. Pat. Application Ser. No. 352,498 filed May 16, 1989. This control apparatus does not use clutch control patterns or compensating data maps, but is adapted to control the lock-up clutch, according to calculated degrees of satisfaction of predetermined fuzzy set control rules for selectively engaging and releasing the lock-up clutch depending upon the running condition (running parameters) of the vehicle. Namely, the lock-up clutch is placed in one of the engaged and released positions which is selected based on the calculated satisfaction degrees of the fuzzy set control rules. In this arrangement, the required amount of control program data is proportional to the number of the control parameters (vehicle running parameters), and is therefore comparatively small, whereby the control apparatus may be simplified and available at a relatively reduced cost.
In the above arrangement, however, the required amount of control program data is larger than that in the arrangement using only the clutch control patterns, in the case where the number of control parameters for selecting the operating positions of the lock-up clutch is relatively small, for example, where only the accelerator pedal position and the vehicle speed are used as the control parameters. Thus, the conventional and recently proposed arrangements are not completely satisfactory in terms of the control reliability and simplicity.